Small photodetectors that are efficient, that can if necessary be tuned for spectral response in specific wavelength regions of interest, and that are compatible with the processing technologies and structures of modern silicon CMOS electronics manufacture are very important for many application areas. Such areas include compact on-chip spectrometers for lab-on-chip type systems, detectors for free-space wavelength division multiplexing (WDM) systems and multi-spectral imaging detectors.
It is particularly useful in manufacture also that such photodetectors could operate with very thin layers of absorbing materials since thin layers of, for example, silicon semiconductor absorbing material, are compatible with modern silicon manufacture. However, previous approaches to either tunability of spectral response or to the use of only thin layers of absorbing material, have required vertical cavities that require different thicknesses for different spectral responses or horizontal cavities that are necessarily at least several wavelengths in size, making them less desirable for dense photodetector arrays for cameras or for coupling to very small light beams.
For example, in U.S. Pat. No. 5,227,648 a photodiode array having sharp and distinct resonances for each detector element is considered. However, the approach for providing this capability is the formation of vertical resonant cavities using layers with variable thickness across the wafer, which is difficult fabrication in its own right, and which also poses formidable integration difficulties with more conventional processing technology.